Some embodiments described herein relate generally to methods and apparatus for the detection and correction of time skew in an optical transmitter. In particular, but not by way of limitation, some of the embodiments described herein relate to methods and apparatus for the detection and correction of time skew associated with the digital domain, the analog domain and the optical domain in an optical M-ary quadrature amplitude modulation (M-QAM) transmitter.
The optical M-ary quadrature amplitude modulation (M-QAM) scheme with digital signal processing (DSP) is an enabling technique for high bit rate (e.g., 100 Gbit/s and beyond) optical networks. Transmitter side serializer/deserializer (SerDes) and digital-to-analog converters (DACs) are building blocks for spectrally-efficient, multi-level signal generation and spectral manipulation.
Any timing misalignment between the in-phase portion (I tributary) and quadrature-portion (Q tributary) on the transmitter data path sourced from the digital, analog and/or optical domains will penalize M-QAM system performance. Hence, it is desirable for skew correction to contain the I/Q skew to less than the symbol unit interval (UI) (e.g., 10% of UI). This presents challenges for component designers and board designers particularly in the case of pluggable photonics, where the SerDes/DACs are not assembled with the photonic module in the factory, preventing an in-factory calibration and correction of the time skew.
Accordingly, a need exists for methods and apparatus for the detection and correction of time skews that can originate from the digital domain, the analog domain and the optical domain in an optical transmitter system.